Answer:
Historical points are a record of some very old contributions that didn't fit into the new system (it was a bit like putting a video cassette into a DVD player). We didn't want you to lose them, so we created a special category for them. "Historical" contribution points can no longer be obtained, and the number won't change. You can view it as a little contribution nest egg.
If you don't have Historical points on your profile, don't worry. It means all of your points fit right into the other point sections in your stats box.